Known as a conventional hydrogen generator configured to prevent a reforming catalyst from deteriorating while the hydrogen generator is not operating is a hydrogen generator configured to supply a raw material gas to a reformer when the temperature of the reformer becomes a predetermined temperature or lower to purge a reformed gas (see PTL 1, for example). FIG. 4 is a schematic diagram showing the conventional hydrogen generator described in PTL 1.
In FIG. 4, the conventional hydrogen generator includes: a reformer 101 configured to generate a hydrogen-rich fuel gas by using a raw material gas and water; a burner 102 configured to heat the reformer 101; a raw material gas on-off valve 103 configured to supply and stop supplying the raw material; and a fuel gas on-off valve 104 configured to supply and stop supplying the fuel gas discharged from the reformer 101. The temperature of a catalyst of the reformer 101 is increased to and maintained at 600° C. or higher by combustion of the burner 102, and stable steam reforming is carried out in the reformer 101.
While the hydrogen generator is not operating, the raw material gas on-off valve 103 and the fuel gas on-off valve 104 are closed, the supply of the raw material gas and the supply of the water are stopped, and the inside of the reformer 101 is filled with the reformed gas that is a reducing gas. When the reformer 101 becomes a predetermined temperature or lower, the raw material gas on-off valve 103 and the fuel gas on-off valve 104 open, and the raw material gas is supplied to the reformer 101 to purge the reformed gas. After the purge, the raw material gas on-off valve 103 and the fuel gas on-off valve 104 are closed. Thus, the inside of the reformer 101 is filled with the raw material gas. Here, the above-described “predetermined temperature” is a temperature at which a decomposition reaction of hydrocarbon does not proceed and carbon deposition due to the decomposition reaction does not occur. For example, if the hydrocarbon is methane, the predetermined temperature is from about 600° C. to 400° C. If the hydrocarbon is the city gas, the predetermined temperature is lower than the predetermined temperature in the case of methane. This is because the city gas contains hydrocarbon including two or more carbons. Specifically, the predetermined temperature in this case is set based on, for example, a preliminary experiment.    PTL 1: Japanese Laid-Open Patent Application Publication No. 2004-307236 (pages 4-6, FIG. 1)